174 GENERAL ZOOLOGY 



types of live stock. From each generation the stockman 

 selects his best cattle or his best pigs as breeding stock. By 

 best he means those which come the nearest to his idea of 

 what a perfect animal should be. Thus if he is raising cat- 

 tle chiefly for meat, he selects the largest animals. On the 

 other hand, if he is interested in producing milk, he chooses 

 those animals which yield the most or the best milk, with 

 little regard for the size or form, which concerned the meat 

 producer. His choice of breeding stock is called artificial 

 selection. In this manner, beginning back before the dawn 

 of history, man has gradually improved his domestic ani- 

 mals. In consciously selecting desirable traits for his breed- 

 ing stock he has at the same time eliminated many 

 undesirable characteristics from his flocks and herds, for 

 desirable as well as undesirable traits are passed on from 

 generation to generation by heredity. 



New races and breeds of domestic animals (Fig. 97) 

 differing in many respects from each other are being pro- 

 duced continuously. Some of these have existed for a long 

 time, others have been produced within the past few years. 

 A Percheron horse and a thoroughbred saddle horse are 

 extremely different in appearance. But we know that man 

 by his own choice has produced these and many other breeds 

 of domestic animals by selecting from stocks that were 

 originally alike. Most of the many breeds or races of do- 

 mestic animals breed true. This means that two individuals 

 of the same breed produce young like themselves except for 

 that degree of ordinary variation which marks all individ- 

 uals. Occasionally sports, or mutants, utterly different 

 from their relatives in certain respects make their appear- 

 ance in breeds of domestic animals. Some of these muta- 

 tions are hereditary and are utilized to change the character 

 of a breed. Hornlessness in cattle is a condition that has 

 existed for a long time in some breeds, and in other breeds 



